The present invention relates to Brassica plants, in particular Brassica oleracea plants which are resistant to Thrips tabaci and herein display agronomically desirable traits.
Thrips tabaci (also known as Onion thrips, or simply thrips) is a highly polyphagous insect and considered as an economically harmful pest for many cultivated crops especially for crops from the Liliaceae, Solanaceae, and Brassicaceae plant families, such as onion, leek, tomato, cabbage, but also in crops from outside these families, such as e.g. cucumber, melon and snap beans (Toda & Murai, 2007, Applied Entomology and Zoology 42: 309-316; Trdan et al., 2005, African Entomology 13: 85-95).
Thrips harm the plant directly by feeding on the plant tissue and indirectly by being a vector for lethal plant viruses such as tomato spotted wild virus (Toda & Murai, 2007, Applied Entomology and Zoology 42: 309-316). Thrips occur worldwide and due to climate change and trade the frequency of thrips infestation is increasing (Trdan et al., 2005; African Entomology 13: 85-95).
Cabbage plants (Brassica oleracea var. capitata L.) belong to the plant family Brassicaceae. The family has a cosmopolitan distribution and consists of approximately 3500 to 4000 species. The family and especially the genus Brassica contains many agronomically important crops such as broccoli, Brussels sprouts, cauliflower, Chinese cabbage, curly cabbage, kale, kohlrabi, mustard, oxheart cabbage, radish, rapeseed, red cabbage, Savoy cabbage, turnip, and white cabbage.
In tissue of Brassica oleracea var. capitata plants affected by thrips callus growths will form. Over time these callus growths become brown making it necessary to remove several layers of leaves from the cabbage head before marketing. Even in storage the development of symptoms caused by thrips may increase. The reduction in head size and weight ultimately leads to a loss in yield. For Brassica oleracea var. capitata it is estimated that 75% of its total acreage suffers from thrips.
Controlling thrips in cabbage by means of applying insecticide is considered environmentally unfriendly and ineffective, because the closed leaves of the cabbage heads provide protection to the thrips, and because thrips are hard to detect, sometimes even only at harvest stage. Thrips infestation in cabbages may occur from the second exterior leaf up to the fifteenth exterior leaf, while insecticides are usually only effective up to the sixth exterior leaf of the head (Trdan et al., 2005; African Entomology 13: 85-95). Therefore, there is an urgent need for cabbage plants which are resistant against this pest.
Although no highly thrips resistant Brassica oleraceae varieties are known, huge differences in susceptibility exist among varieties. Certain varieties show an intermediate level of resistance, but no varieties are known that are highly resistant against Thrips tabaci. The genetic background of this intermediate resistance against thrips is still poorly understood and it is thought that this is a polygenic trait, inherited as a gene complex in which many genes are involved (Voorrips et al., 2008; Euphytica 163: 409-415).
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.